Copper coil turns in generator fields are insulated from each other by electrical insulating sheet like materials or heat resistant calendered papers. Typically, the sheet materials include a cured polyester woven glass cloth or a cured epoxy woven glass cloth material or NOMEX™ (insulating material) aramid paper that has been densified by calendering. Dielectric films with high glass transition temperatures may also be used. The insulating materials are normally coated on one side with a thin adhesive film, typically an epoxy adhesive, that cures when subjected to heat and pressure to bond the insulating sheet material to the copper.
There are several problems with this known process. First, the adhesive on the woven glass cloth sheet material or the aramid paper consists of a dry, thin coating that has virtually no adhesion or “grab” to the copper coil turns during manufacture or assembly of the generator field. As a result, heretofore, it was necessary to use a strip of adhesive transfer tape to hold the insulation in place during the field assembly. However, conventional adhesive transfer tapes do not cure during bake (causing the adhesive in the tape to solidify). The transfer tape thus becomes a permanent constituent of the field coil insulation adhesive system. During operation of the generator, the heat created by the generator weakens the bond of the transfer tape adhesive to the copper and can lead to slipping of the insulation or even eventual shorting of the coil turns resulting in reduced generator performance, or generator failure.
Another known problem with prior art insulating materials is that epoxy adhesives on sheet materials or aramid paper normally require very high cure temperatures and the large mass of steel and copper in the generator field must be heated to cure the adhesives. Because of the large mass, it can take excessive time to reach the required cure temperature for the adhesives.
Still another problem has been that the electrical insulating sheet-like materials or heat resistant calendered papers must be pre-coated with the adhesive. Depending on the design and rating of the generator, the specified width and thickness of the insulating material can change, resulting in the need to carry a large inventory of adhesive coated insulating materials. Although most adhesives have a relatively long, but finite shelf life, significant costs can be incurred due to having to discard insulating materials that reach the end of their useful shelf life before they can be used.
In addition, the manual application of adhesive to the insulation prior to assembly has always been labor intensive, resulting in unintended and unwanted bonding of surfaces that move relative to one another during operation of the generator. Further, in the past a waiting period was often required to allow solvent to evaporate or to allow the adhesive to partially cure to make it more “tacky” before proceeding with installation of the next piece of insulation. Thus, the assembly time for systems using conventional adhesives could be quite long.
More recently, a commercial Nomex™ aramid paper based tape has been used that includes a thermosetting synthetic resin adhesive. This product has the advantage of being pressure sensitive at ambient temperatures to secure the insulation to the copper at ambient temperatures. However, the required cure temperature was at least one hour at 150° C. or two hours at 130° C. In addition, although this material exhibited good adhesion at room temperature, the bond strength dropped precipitously at about 80° C. with little or no bond strength at higher temperatures. There was also no discernible improvement in bond strength at elevated temperatures when the cure time or cure temperature was increased. These deficiencies made the Nomex™ type of tape unacceptable for use in generator fields.
Another advance in copper bonding technology involved the application of a heat curable adhesive material to one surface of the insulation. However, the adhesives had to be heated by baking the field to relatively high temperatures to achieve the desired bond strength. Such systems normally required cure temperatures from 130° C. to 160° C. and bake times from 10 to 12 hours. Although some thermoset adhesives were developed with lower cure temperatures, none cured at or below about 100° C., i.e., low enough to benefit manufacturing. The adhesives were also tack-free on the surface and thus required a pressure sensitive transfer tape to hold the insulation in place during assembly. Since the pressure sensitive transfer tape could not be removed from the turns at the conclusion of the assembly process and the tape does not cure to a thermoset solid during bake, it remained a significant weak point in the bonding of the insulation to the copper turns.
Thus, a significant need still exists for a resin composition that enables the manufacturing of insulating films, sheet materials and calendered aramid papers with a thin coating of a pressure sensitive adhesive that bonds at ambient temperatures. A need also exists for a pressure sensitive adhesive capable of curing at elevated temperatures to a thermoset solid with good adhesive bond strength at operating temperatures up to at least 160° C.